The present invention relates to a hot-melt adhesive interconnector and a method for the preparation thereof. More particularly, the subject matter of the present invention is a hot-melt adhesive-type interconnector used for electrically connecting two sets of electrodes, for example, on circuit boards facing each other with high reliability even when the thus connected assemblage which has electrodes arranged at a fine pitch is used at an elevated temperature.
It has been eagerly desired in recent years along with the rapid progress in the electronic technology to develop a simple and reliable method for electrically connecting lead-out electrodes which function as the terminals of a glass substrate for various kinds of functional units such as liquid-crystal display units, plasma display units and the like and the terminal electrodes of a printed circuit board with a patterned copper foil on a rigid plastic material used for driving or controlling the performance of the functional unit.
In view of the restriction that the circuit pattern and lead-out electrodes, which are formed, for example, of an ITO film having transparency, have no receptivity for solder, one of the conventional methods for obtaining electrical connection between the electrodes is the use of an anisotropically electroconductive adhesive composition as is schematically shown in FIG. 1 to illustrate the principle. Namely, an end portion of a flat cable having a pattern of conductors 22 to serve as the electrodes formed at a uniform pitch on an electrically insulating film 21 made of polyester, polyimide and the like is mounted on a glass substrate 23 having lead-out electrodes 24 with a gap between the flat cable 21 and the glass substrate 23 or, in particular, between the electrodes 22 of the flat cable and the lead-out electrodes 24 on the glass substrate 23 filled with an anisotropically electroconductive adhesive composition which is a dispersion of particles 26 of an electroconductive material such as metals and carbon in a matrix of an electrically insulating adhesive resin 25 in such a density that the conductive particles 26 are not in contact with each other as far as possible so that electrical connection is established between the electrodes 22 and 24 via the conductive particles 26 interposed therebetween. The other end portion of the flat cable is connected to the lead-out terminals of a printed circuit board in the same manner as above thus to establish electrical connection between the glass substrate and the printed circuit board.
The above described method using an anisotropically electroconductive adhesive composition has several problems when the method is applied to make an electrical connection between the end portion of a flat cable and the set of lead-out terminals on a substrate plate as a consequence of the principle of the method in which a single or a few number of conductive particles 26 are interposed between the oppositely facing two electrodes 22 and 24. Namely, the electrodes 22 and 24 on the flat cable and the substrate are jointed together by the van der Waals force acting between the insulating resin 25 and the surfaces of the electrodes 22,24 to maintain the electrical connection. Accordingly, the electrical connection between the conductive particles 26 and the surfaces of the electrodes 22,24 is very unstable because of the possible intervention of an insulating thin film therebetween. Since the anisotropically conductive adhesive composition contains conductive particles 26 having different particle sizes even if the powder has been subjected to particle size classification to obtain a uniform particle size distribution, the connection formed with a larger particle may act to pull apart the connection formed with smaller particles to locally destroy the electrical connection at an elevated temperature of, for example, 70.degree. C. or higher where the matrix resin is softened. Therefore, it is sometimes difficult to obtain an assemblage which satisfies certain standards of products which require a test for the reliability of performance at 85.degree. C.
When the conductive particles dispersed in the anisotropically electroconductive adhesive composition are too fine or the particle diameter is smaller than, for example, 10 .mu.m, the thickness of the layer of the adhesive between the electrode surfaces must be correspondingly so small that sufficiently high adhesive bonding strength can not be obtained. When a relatively coarse conductive powder is used, on the other hand, troubles are sometimes encountered especially when the pitch of the electrodes is very fine due to the failure of electrical insulation between the adjacent electrodes on the same substrate because the coarse conductive particles are positioned not just on the electrode surfaces but also in the spaces between the electrodes and can be brought into contact with each other within the plane parallel to the surface of the substrate. This situation leads to a limitation that arrays of electrodes at a pitch of 500 .mu.m or smaller cannot be electrically connected by use of an anisotropically electroconductive adhesive composition.